1. Field of the Invention
The invention relates to a color-picture projection system provided with a plurality of optical channels which each comprise a monochrome picture display tube, and a projection lens system for projecting the picture of the display tube onto a screen which is common to all the channels and on which the individual pictures are superimposed to form one color picture, a plurality of reflectors being arranged between the projection lens system and the projection screen for folding the radiation path. The invention also relates to a projection lens system intended for use in such a color-picture projection system.
2. Description of the Prior Art
Such a color-picture projection system is disclosed in, for example, U.S. Pat. No. 4,400,723. The system described therein comprises three cathode-ray tubes which are arranged in line and which produce a blue, a red and a green picture respectively. Magnified images of each of these pictures are formed on a common projection screen by means of separate projection lens systems. Between the projection lens systems and the projection screen there are arranged, in this order, a first mirror which reflects the beams emerging from the the projection lens systems in an upward direction and a second mirror which reflects this reflected beam towards the projection screen in a substantially horizontal direction. By means of these mirrors the radiation path between the projection lens systems and the projection screen is "folded", i.e. accommodated within a smaller volume without thereby reducing the length of this radiation path, which length must be substantial in view of the required magnification.
The elements of projection lens systems employed in current color-picture projection systems are preferably made of a transparent plastic. These projection lens systems have a substantially lower weight and smaller dimensions than projection lens systems employing glass elements. Projection lens systems comprising plastic elements, however, are only capable of forming a sharp image by means of a substantially monochromatic beam, or in other words, these projection systems exhibit chromatic aberration. Projection lens systems comprising glass elements can be rendered achromatic to minimize chromatic aberration, but this requires additional steps such as the use of different types of glass for the various lens elements.
The phosphors which are suitable for use as light-emitting layers in cathode-ray tubes because of their high luminous efficiency have a given spectral width, which means that upon excitation by an electron beam they not only emit pure red, green or blue light, i.e. light of one specific wavelength, but also light components of wavelengths in a range around the desired wavelength. The last-mentioned components cannot be imaged sharply on the projection screen by the relevant chromatic projection lens system. On the three sharp images in the colors red, green and glue projected on this screen spurious unsharp pictures of different colors are superimposed. As a result of this the sharpness of the color picture formed on te projection screen will not be optimal.
The spurious images on the projection screen may be reduced by arranging a transmission filter which only transmits the desired wavelength and which absorbs other wavelengths in the optical channels between the projection lens systems and the projection screen. However, such filters only permit a reduction of the spurious images and not an effective elimination because the transmission-versus-wavelength characteristic of such a filter is not sufficiently steep. Moreover, the use of such a filter in an optical channel means the introduction of an additional element and of two additional surfaces which may produce reflection.